It has been found that considerable quantities of rain water enter sanitary sewers through the openings conventionally provided in manhole covers. Such covers may or may not have holes specifically for venting gases, but in any case were provided with holes for lifting tools so that they might be removed for access to the sewer. In addition, rain water can enter around the edge of a manhole cover and get into the sewer, because metal to metal contact usually is not sufficiently waterproof to keep it out, particularly if the manhole cover and cover frame have been in service for some time.